The present invention relates to a process for preparing polyethylenes having a wide molecular weight distribution and superior in melting characteristic and moldability. More particularly, it is concerned with a process for preparing polyethylenes superior in performance by combining a specific catalyst with a two-stage polymerization process.
In general, polyolefins used for obtaining molded or formed articles such as bottles, cable conduits and ultra-thin films are required to fully withstand molding or forming conditions in plastisized state and be formed into desired shapes easily. This requirement may be satisfied by the use of a polyolefin having an increased melt index (a lowered average molecular weight). Such a polyolefin, however, can only afford a product inferior in strength, e.g. impact resistance and tensile strength. On the other hand, a polyolefin having a low melt index afford a product superior in strength, which product, however, is inferior in moldability. It is known that this problem can be solved by using a polyolefin having a wide molecular weight distribution.
Moreover, physical properties required for polyethylenes have been diversified recently, and also from the standpoint of resources saving there is a tendency to using resin in an amount as small as possible in a range not impairing physical properties. For example, as to bottles and films, there is now a tendency to make them as thin as possible while maintaining their strength. A polyethylene which even in a small amount exhibits good processability and high impact strength, tensile strength and resistance to environmental stress cracking is keenly desired.
Several methods have been proposed as methods for preparing polyethylenes having a wide molecular weight distribution. According to one proposed method, a high molecular weight polyethylene and a low molecular weight polyethylene are mixed together (see Japanese Patent Publication No.3215/1970 and Japanese Patent Laid Open Nos.100445/1979 and 133136/1982). A two or more multi-stage polymerization method has also been proposed (see Japanese Patent Publication No.11349/1971 and Japanese Patent Laid Open No.19788/1977).
Polymers prepared by such proposed methods are wide in molecular weight distribution and fairly good in resistance to environmental stress cracking (hereinafter referred to as "ESCR"), but are poor in melt viscoelasticity during molding, and molded products obtained using such polymers are apt to have fish eye or gel.